The extrinsic fluorescent probe, merocyanine 540 (MC540), has been reported to be specifically taken up by leukemic lymphocytes. We are investigating the potential use of this compound as a marker for transformed lymphocytes in spleen and lymph nodes. By using a fluorescence-activated cell sorter (FACS), we are attempting to isolate the stained cells for further microscopic study. Using dual fluorescent labeling techniques, we will also determine whether all subsets of transformed lymphocytes react with MC540, or whether MC540 uptake is limited to a population of these cells. Finally, we feel that the degree of MC540 staining may be related to lymphoma subtype. Preliminary data suggest that nodular lymphomas stain weakest, followed by the mixed cell type, with lymphoblastic staining the strongest. We are currently extending these observations.